The present invention relates to a backlighting device for an information display element of a portable object such as a wristwatch. The backlighting device comprises a light guide plate having a front face and a rear face, a light reflector located on the rear face of the light guide plate and a source of visible light. This source of visible light feeds light beams at the edge or boundary of the light guide plate. These light beams spread in the light guide plate between its two faces to be directed for illumination of the display screen located above the front face of the light guide plate.
In the case of an object such as a wristwatch, the display element is understood to be a watch face, on which the hands indicate the time, a liquid crystal-type display screen, where time, text or image information can be displayed, or a combination of watch face and screen.
Devices for illumination from the back, which are referred to as backlighting devices, for display screens in particular of the liquid crystal type have already been described in the prior art. Such devices are encountered in particular in objects such as cordless mobile phones, portable computers or other types of portable objects in current use.
In these objects of relatively large dimension, it is generally simple to illuminate a display element uniformly from the back over its entire visible surface. To do this, illumination from the back is achieved by one or a plurality of light sources disposed to face layers of a light guide plate. These light sources feed light beams into the light guide plate so that the light is directed towards the display element. With several light sources spaced at a relatively substantial distance in relation to the visible portion of the display element, the display element can be illuminated relatively uniformly.
In the case of a wristwatch, it is difficult to provide several light sources to illuminate the display element from the back, since the area available inside the case is limited. Since the number of components to be mounted in said case must be reduced to fulfill all the desired functions of the watch, a single light source is generally provided to ensure illumination of the display element.
In the case of a traditional backlighting device, this light source is mounted in the case of the watch below the display element in a position close to the visible portion of the display element. Even if the light source is partially concealed below the display element, its radiation of light causes a halo around the position of the light source, which is visible to the outside via the display element.
Illumination of the display element from the back is therefore not uniform, since the light intensity of this halo is greater than the light intensity illuminating the rest of the display element. This is major disadvantage of such a backlighting device, if it is provided, for example, to fit an upmarket type of watch.